Thread: Brake troubles

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Navy-maybe you missed it in my post-you don't need to move the front calipers to the rear to see if they work proving that your rears are the culpred---just change the lines at the master or combo valve and then see if the fronts still work-that will prove/disprove wether its calipers or master issue. However, since neither rear works now, its probably something thats common to both that is wrong and if when you hook the fronts up you'll prove it.

I agree with Jerry simple & easy, swapping questionable parts with known good eliminates one or the other, just swap them at the mc will work well. If those calipers don't collapse when hooked to the known good side of the mc you have 1 of 2 things that I can see.
1. Siezed brake piston
2. Too big a bore on the rear caliper, you can't move enough fluid to collapse the rear without the fronts collapsing fully. This could be easily confirmed by c-clamping the piston into the caliper and taking some calipers and measuring the inside bore or the outside of the piston of each and confirming fronts and rears are the same.

If either one is the case like mentioned by others you could always swap out the calipers with the same front calipers using them on the rear (no ebrake but your state inspection might require it, my state doesn't might have to use a pinion mount type e-brake) or replace the front calipers with bigger bore calipers that match the diameter of piston of your rear calipers, the gm metrics are available in many different bore sizes to "tune" a system from Speedway.
If the piston is siezed get new calipers.
We are staying tuned here, we are all curious!

Originally Posted by jerry clayton

Navy-maybe you missed it in my post-you don't need to move the front calipers to the rear to see if they work proving that your rears are the culpred---just change the lines at the master or combo valve and then see if the fronts still work-that will prove/disprove wether its calipers or master issue. However, since neither rear works now, its probably something thats common to both that is wrong and if when you hook the fronts up you'll prove it.

I have already switched lines on the MC and that had no effect on the rears I never did try the fronts to see what they do with the lines switched, I can only turn them by hand and it doesn't take much to stop them. With todays MC change I doubt its the master cyl. I will run pressure test tomorrow on both ends and see what's what. I hope. Thanks again for your help.

Originally Posted by Matthyj

I agree with Jerry simple & easy, swapping questionable parts with known good eliminates one or the other, just swap them at the mc will work well. If those calipers don't collapse when hooked to the known good side of the mc you have 1 of 2 things that I can see.
1. Siezed brake piston
2. Too big a bore on the rear caliper, you can't move enough fluid to collapse the rear without the fronts collapsing fully. This could be easily confirmed by c-clamping the piston into the caliper and taking some calipers and measuring the inside bore or the outside of the piston of each and confirming fronts and rears are the same.

If either one is the case like mentioned by others you could always swap out the calipers with the same front calipers using them on the rear (no ebrake but your state inspection might require it, my state doesn't might have to use a pinion mount type e-brake) or replace the front calipers with bigger bore calipers that match the diameter of piston of your rear calipers, the gm metrics are available in many different bore sizes to "tune" a system from Speedway.
If the piston is siezed get new calipers.
We are staying tuned here, we are all curious!

The pistons are not seized I know that for sure. The caliper bore size now that another question all together. Looking at the fronts and rear calipers it would seem to me that the fronts hold more fluid because of the E-Brake stuff that's in the rear caliper . Physically they look very close other then the e-brake.
Thanks for your help I hope after testing tomorrow I'll have an answer.

OK----one other thing-I have always said about brake issues-but it was generally about braking from speed but I do see how it could be possible in this case-------when you have a braking problem-look for the cure at the other end of the car-----Maybe the choice of either the front and or rear calipers is the wrong size ratio of area/volume. It could be that the fronts are too small and your master cylinder is being stopped at that point before the rears do proper contact-OR, the rears are too big and you don't have enough volume for them?? or both??? Since they went to the dual brake anti fail system back in 1967?(I think) the volumes and pressure are strickly regulated to get a certain reaction from your brakes for if one end of your vehicle fails, the other end will stop you BUT there will be severe symthons from pedal pressure and travel.

As this gets more and more complex I would resort to a very basic method of chasing down where the faults lie-I would procure a simple master for a single brake port (maybe clutch master?) and with a pressure guage to verify pressure would then check ONE caliper at a time to prove each separate part/ component and then go deeper as I put the complete system together.I'd remove all the valves, combo, residual, delay, porportional, etc and only work with pedal, manual master,etc, etc


If you don't have it worked out by the time that plane is ready to fly , I'll come down ??????????????Selma Texas?? I remember Selma, Ala 1965

Suggest you go back and re-read the instruction sheet that Matt posted regarding the symptom if you don't get the sprag unit properly adjusted. Seems to me that's a perfect description of your problem, but I may be missing something in what you said. Also, your rear calipers are designated L & R. Is it possible for them to be reversed and still hook to the cables and function? The sprag units are threaded LH/RH to "unscrew" tighter with application of the e-brake lever, as I understand.

Originally Posted by jerry clayton

I'd remove all the valves, combo, residual, delay, porportional, etc and only work with pedal, manual master,etc, etc

Sounds right.

.

Originally Posted by rspears

Suggest you go back and re-read the instruction sheet that Matt posted regarding the symptom if you don't get the sprag unit properly adjusted. Seems to me that's a perfect description of your problem, but I may be missing something in what you said. Also, your rear calipers are designated L & R. Is it possible for them to be reversed and still hook to the cables and function? The sprag units are threaded LH/RH to "unscrew" tighter with application of the e-brake lever, as I understand.

Originally Posted by jerry clayton

As this gets more and more complex I would resort to a very basic method of chasing down where the faults lie-I would procure a simple master for a single brake port (maybe clutch master?) and with a pressure guage to verify pressure would then check ONE caliper at a time to prove each separate part/ component and then go deeper as I put the complete system together.I'd remove all the valves, combo, residual, delay, porportional, etc and only work with pedal, manual master,etc, etc

I can't disagree with Jerry's approach of proving each component one by one, using a single port master, etc, and I expect it could be done on the bench with a hand full of pre-formed brake lines and simulated rotors, but it seems that you ought to be able to slay the beast on the car. One other thing, it's my impression that you've got to work both rear calipers together unless you take one out and plug that line. I believe that even one rear caliper with the sprag unit down in the bore will "rob" volume from the system and keep you from generating full line pressure.
Good hunting, Navy!

In post 34 you state that with the front brakes connected that the pedal push rod only moves about 3/4 inch but without the fronts hooked that it moves 1 1/4 inches-that says "the brake caliper volume requirements aren't compatable with the system--front calipers should take more volume than rears in a properly sized system---Your combo valve I haven't seen one like it????

Originally Posted by jerry clayton

In post 34 you state that with the front brakes connected that the pedal push rod only moves about 3/4 inch but without the fronts hooked that it moves 1 1/4 inches-that says "the brake caliper volume requirements aren't compatable with the system--front calipers should take more volume than rears in a properly sized system.

I believe that this situation would be caused by the rear sprags not coming out of their "pockets", increasing the volume capacity of the rear calipers. The more we talk about these "e-brake rear calipers" the less I like them.

Not trying to re-post what has already been posted, just trying to simplify a step by separating it;
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Roger, I would almost agree with you but I put the kit on from MPB and it could not have been any easier, I have zero issues, now keep in mind 80% of braking is done by the front and my rears could be siezed tighter than a drum and I might not know it yet!
As for Navy, I still say either incorrect volume (too big) on rears or sprag is siezed.
1.) Volume issue, you could not probably physically see the size difference, heck a Wilwood is twice the physical size of a GM and holds way less volume even with 4 pistons so, you could take one of the front calipers off swap it with the one of the rear still, don't hook up of e brake and hit the peddle if this is the case, on the front brakes the smaller volume caliper would collapse and the other larger on the front would not, problem either solved or eliminated about 15 minutes on a bare frame as the caliper mounts are the same for these. if they both stop the rotors while pressure is applied when you are swapping them back simply crank the e-brake arm and see if they are ractheting.
I know this to Navy is about 6 guys barking commands from their easy chairs but this can be figured out and eliminated pretty easy, if both collapse at the same time when you have a rear and a front caliper on the front and then find they are ratcheting I would then start eliminating valves etc. But swapping calipers is free (no fittings to buy) and would also confirm or check the valves to some extent since both calipers could be compared on one end of the vehicle with the same valving.

OK the results are from pressure testing

Without engine running at the MC 100-150 psi
With engine running At the MC 600 psi
lines to front 600 psi each side
line to rear 600 psi
With a 3 foot cheater bar on the brake pedal I reached 900 psi at rear with lots of pushing on the bar which was way over and beyond anything anyone could do with foot power on the pedal.
I did a search and found that 900 + psi seems to be the norm.

IS IT POSSIBLE THAT THE DARN POWER BOOSTER IS BAD ? Its new but ?

In post 34 you show a 33* angle in the brake pedal arm---and stated later that you had 3/4 inch travel in master rod-----thats not a very good mech leverage ratio-looks like maybe a 3 inch arm and a 7-1 would get it out to 21 inches? 5-1 maybe 15? you can raise the pressure by a better leverage/geometry and even a smaller diameter bore master. maybe down to 7/8? there are graphs out there that show the volumes/pressure differances-------

Keep in mind there is a difference in pressure & volume, If you unhooked the caliper and put a gauge on the front and back and got the same pressure (you did) that means that you still could have a volume issue or stuck piston, as of now you know that the fronts collapse at 600 psi and the rears don't, I still believe the easiest and no cost option would be to swap calipers on one side, those fronts could collapse at 600 psi as could the rears but the volume could be to much of a difference to ever get full pressure on the rear, it would also check your plumbing (valves, etc) if the rear don't collapse on the front rotor and the front does on the rear you now simply have a stuck piston or volume issue, and all else is kosher.
It doesn't mean your booster is bad either. A person can exert alot of force in a panic stop when his back is against the seat and he's using his legs, keep in mind that 99% of the time you don't exert full force on a brake pedal because you don't normally panic stop (normally), pedal leverage as Jerry said is mechanical advantage, as well as a smaller diameter master cylinder normally the pedal ratio is different for power brakes than non power.